1. Field of the Invention
This invention relates to a process and apparatus for the production of combustible gas from waste substances and other combustible materials which, because of their composition, e.g. their water content or pollutant combustion residues, have an adverse effect on, or are unsuitable for, direct incineration, e.g. domestic and industrial refuse, used oil and petroleum, wood and wood waste, lignite, peat and other organic substances, rubber, plastics and so on.
2. Description of the Prior Art
A number of different processes have been disclosed for the utilisation of waste substances and other such materials, wherein the predried charge is burnt at temperatures far above 1000.degree. C. in rotary kilns, shaft furnaces or the like, the resulting heat being used predominantly for steam generation or for central heating.
Known plants for the incineration or gasification of refuse or other waste substances have a number of disadvantages. The waste gases forming in large quantities because of the large excess of air must be purified, and this is an expensive business which is nevertheless only incomplete despite expensive filter and scrubbing plants. Re-usable constituents, such as scrap iron and non-ferrous metal, are lost because of the high process temperature of above 1000.degree. C. The resulting slag consisting of ash and broken glass, together with the included salts, may pollute groundwater or other effluents. The resulting heat is in the form of steam, which can be used only to a limited degree.
Processes for the production of a combustible gas from waste substances and other carbon-containing materials which are unsuitable or only inadequately suitable for direct combustion have long been known. In one method of this kind (Austrian Pat. Spec. No. 44467) the waste is burnt in a shaft furnace and for oxygen enrichment purposes the escaping gases are passed through another furnace filled with red-hot coke. Two alternately operated coke ovens are used to perform a continuous process, the waste gases escaping from whichever coke oven is being blown being fed to the combustion oven for the waste and being passed, together with the smoke or low-temperature carbonisation gases expelled from the waste, through the hot coke in the other coke oven. In this process, therefore, there is no real low-temperature carbonisation of the waste substances; instead they are burned at high temperatures so that the combustion gases produced from the waste contain only small quantities of reactive constituents and the combustible gases generated have only a low heat content. A disadvantage in terms of operation is that the combustion of the waste substances at relatively high temperatures results in the loss of the re-usable substances and at these combustion temperatures the non-combustible constituents in the waste materials fuse together or sinter as a result of their sometimes low softening temperatures, and thus clog the shaft furnace.
In another known process for the production of a fuel gas free of carbon monoxide, from refuse and waste substances, with the simultaneous production of cyanogen compounds (Austrian Pat. Spec. No. 1,664), the waste materials are heated to temperatures of between 800.degree. and 1000.degree. C. for partial conversion to smoke gases which are fed to a shaft reactor filled with reactive materials in a specific arrangement. In this shaft reactor the smoke gases are passed over hot paper ash to bring them to dissociation temperature and are passed over coke or other carbon-containing material in the dissociated state. The conversion of the smoke gases formed previously by the waste distillation takes place in these conditions. Like the first-mentioned process, this process is also unsuitable for the use of domestic or industrial waste and a number of other carbon-containing materials, because of the high gasification temperature of 800.degree. to 1000.degree. C., the non-combustible constituents automatically sintering together. Charging the shaft reactor with paper ash and coke in separate layers means a considerable outlay and the combustible gas obtained has only a low heat content because of the high gasification temperature at which a considerable proportion of the gases is already burnt. Other known gasification processes are described in German Pat. Specs. Nos. 972,468, 672,921, 585,274, 445,334, and Austrian Pat. Spec. No. 1,654,942.
A common disadvantage of all these known processes for the production of combustible gas is that the low-temperature carbonisation temperature of the materials used is about 1000.degree. C. so that only materials having a high softening point for their non-combustible constituents can be gasified. Furthermore, in the case of domestic or industrial waste, for example, this high gasification temperature means that the oxygen reaching the gasification reactor together with the waste substances forms permanent compounds with the low-temperature carbonisation gases and these compounds cannot be converted in the following reaction bed so that the combustible gas produced is of low energy and the effective conversion efficiency is low.